Because of the interest in producing ethanol from the five as well as the six carbon sugars in biomass, it would be useful to have yeasts which ferment both sugar types. However, while many yeasts ferment hexoses, they are usually considered to be unable to ferment aldopentoses. Several species ferment a ketopentose (P. Y. Wang, C. Shopsis and H. Schneider, Biochem. Biophys. Res. Comm., 94, p. 248-254, 1980), but biomass pentoses are aldopentoses, e.g. D-xylose.
A process has been described which depends on the addition of glucose isomerase to culture media containing D-xylose to form D-xylulose, which is then converted to ethanol by several yeasts (Wang, Johnson and Schneider, Biotechnology Letters 1980, Vol. 2(6), p. 279-284). A similar process has been reported by others (C.-S. Gong et al, Applied and Environmental Microbiology, Vol, 41, No. 2, Feb. 1981, p. 430-436). This process is limited by the requirement of relatively long culturing times and the high cost of enzyme.
One report has been noticed where some D-xylose was converted to ethanol by Candida tropicalis (H. Karczewska, Compt. Rend. Lab., Carlsberg, 1959, Vol. 31, p. 251-8). The yields of ethanol were unclear.